The process of resistance spot welding has become one of the most widely accepted means of joining metallic parts. Typically, the materials to be joined are placed between two water cooled copper alloy electrodes. A very high current is then passed between the electrodes. As the current flows through and between the workpieces, heat is generated. The heat results in the formation of a weld "nugget" at the "faying" interface between the two workpieces.
The nature of mass production spot welding requires the ability to make thousands of welds without machine readjustment. For example, the average automobile contains more than 5000 resistance spot welds. In automated welding the current, electrode force and welding time may be set at desired levels using machine controls, however, electrical resistance is a property of the material being welded. As the weld develops, the resistance varies dynamically. Versatility is afforded in resistance spot welding by choosing between low current, long time welding and high current, short time welding. Thus, a range of acceptable welding conditions exists for any material, given its composition and thickness. The maximum current for a given weld time is determined by the expulsion of liquid metal between the sheets being welded. The minimum current is determined by the minimum acceptable nugget diameter. Both expulsion and small weld nuggets produce welds with inferior mechanical strength.
To allow for parameter variations during a welding run, it is desirable to use material with the greatest welding current range. While welding time is accurately controlled by modern welding machines, welding current density is a much more difficult parameter to control. This is due to machine disparities, "mushrooming" or flattening of the electrode tips, and to resistance variations in the sheet metal. Because of this, some manufacturers require that sheet steel demonstrate an acceptable welding range of several thousand amperes at 12 cycles.
Traditionally, manufacturers have relied upon low-carbon sheet steel as their welding material because of its superior characteristics. There is about a 1000.degree. F. difference between the softening and melting temperatures of low carbon steel, permitting it to be weldable over a wide time and current range. For comparison, aluminum has a short plastic range of less than 300.degree. F., making it more difficult to weld successfully.
Prompted by the incentives to produce lighter articles, manufacturers, particularly automakers, find themselves no longer using only low carbon sheet steel. New alloy developments now offer the substitution of higher strength to weight ratio, low alloy, (HSLA) steels. These steels, which increase their strength through compositional variations, also exhibit a marked decrease in their resistance spot weldability.
Therefore, there exists a need to improve the weldability of materials, particularly (HSLA) steels and other materials which presently exhibit a narrow range of acceptable welding currents and time periods.